Murine typhus is unique among the major arthropod-borne infections in that it can be a household infection because of its intimate association with commensal animals and their fleas. Existing knowledge does not adequately explain many important features and mechanisms which are involved in the extraordinarily successful spread, establishment and maintenance of this zoonosis. This research proposal will examine, under controlled conditions in the laboratory, using quantitative techniques, selected components and interactions of the rickettsia-vector complex. The project goals are: (1) Molecular characterization of ELB to understand its taxonomic and biological relationships with the members of the typhus group and in particular, Rickettsia typhi. Studies include sequencing of 16s rDNA and PCR/RFLP analysis of selected samples of R. typhi and ELB isolates obtained from opossums and humans. Also included under this specific aim are the isolation of ELB agent from the infected human and opossum blood specimens, establishing ELB infection in cultured mammalian cell lines, laboratory rats, and experimental infection of the cat flea, Ctenocephalides felis; (2) To determine the degree of interplay between the R. typhi and ELB agent in arthropod vectors and mammalian hosts. In particular, the following studies will be carried out: (A) dynamics of rickettsial infection in the flea midgut epithelial cells, (B) association of cytoplasmic incompatibility in fleas due to germ line infection with R. typhi and ELB, and (G) identification of rickettsial proteins that may be expressed or underexpressed during the rickettsial transition between fleas and mammalian hosts; and (3) Identification, cloning and characterization of the genes encoding R. typhi and ELB 120 kDa surface protein antigens (SPAs). Rickettsial SPAs are the most abundant surface proteins, and have been of interest because of their strong immunogenecity and species-specific antigenic properties. Therefore, limited immunological studies will be performed to determine the degree of cross protection in animals immunized with P. typhi and ELB upon challenge with the heterologous rickettsiae and involvement of 120 kDa in the protection.